Health and Human Services Secretary Kathleen Sebelius testifies on Capitol Hill in Washington, Thursday, April 10, 2014, before the Senate Finance Committee hearing on the HHS Department’s fiscal Year 2015 budget. Sebelius said 7.5 million Americans have now signed up for health coverage under President Barack Obama’s health care law. That’s a 400,000 increase from the 7.1 million that Obama announced last week at the end of the law’s open enrollment period. The figure exceeded expectations, a surprise election-year success for the law after a disastrous roll-out.

WASHINGTON — Health and Human Services Secretary Kathleen Sebelius will step down Friday, and President Barack Obama will nominate his budget chief as her successor, White House officials confirmed Thursday, marking an end to the secretary’s tumultuous five-year tenure.

Sebelius entered the Cabinet as a well-regarded former governor of conservative Kansas but came under intense fire in the fall during the botched rollout of the Affordable Care Act. Obama made it clear to his aides at the time that he would not seek her resignation, although several Democrats had argued privately that someone should be held accountable for the problems with the federal health insurance exchange.

Sebelius remained a tireless promoter of the health care law over the past six months, urging uninsured Americans to sign up on state and federal exchanges.

Traveling to major cities such as Miami, Phoenix and Houston to reach residents whose Republican governors opposed the law, she held 43 local events and more than 100 local interviews over the past several months.

Obama will nominate Office of Management and Budget Director Sylvia Mathews Burwell to take Sebelius’ place. Although Burwell does not have an extensive background in health care policy, she is known for her strong management skills and is popular on Capitol Hill. The Senate confirmed her as OMB director 96-0 almost exactly a year ago.

Bloomberg News and The New York Times first reported the news of Sebelius’ resignation Thursday night.

According to a White House official, Sebelius notified Obama in early March that she planned to leave.

“At that time, Secretary Sebelius told the president that she felt confident in the trajectory for enrollment and implementation of the Affordable Care Act, and that she believed that once open enrollment ended, it would be the right time to transition the department to new leadership,” said the aide, who spoke on the condition of anonymity to discuss internal deliberations.

Other officials said Sebelius timed the conversation with Obama to a moment when she thought that the enrollment troubles had turned around.

Senior Democrats on Capitol Hill appeared to be caught off guard by the announcement, but House Minority Leader Nancy Pelosi of California praised Sebelius’ service.

“From Day One, Secretary Kathleen Sebelius has remained laser-focused on a single purpose: to make health care a right, not a privilege, for all Americans. Her leadership has been forceful, effective and essential,” Pelosi said in a statement.

Although Sebelius is best known as the public face of the Affordable Care Act, her work spanned issues from the H1N1 virus to childhood obesity to parity for mental health treatment.

Congressional Republicans said the larger problem was not Sebelius’ implementation of the law but the legislation itself.

“Secretary Sebelius was asked to promote something unready, poorly structured and unpopular,” Sen. Charles Grassley of Iowa said in a statement. “She was given a law that was just about written in pencil the way the deadlines changed all the time. That put her in a position of having a strained relationship with Congress.”

Despite the troubled launch of healthcare.gov, which left many consumers unable to access the federal exchange for nearly two months, the administration managed to meet its initial goal of enrolling 7 million Americans during its initial sign-up period.

On Thursday, Sebelius announced that about 7.5 million consumers had enrolled, although those who don’t pay their premiums will not be insured.

Sebelius was confirmed April 28, 2009. Her tenure — 1,808 days as of Thursday — is about 500 days longer than the average for HHS secretaries, dating to the Carter administration.

Federal health officials said Thursday night that she has not set a departure date but that her plan generally is to remain until her successor is confirmed.

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